In the 3GPP (3rd Generation Partnership Project), standardization of LTE-Advanced (LTE-A) has proceeded, as the 4G mobile communication system to actualize communications of higher speed and larger capacity than LTE (Long Term Evolution) that is evolved specifications of the 3G mobile communication system. In addition to actualization of high-speed large-capacity communications, in LTE-A, improvements in throughput in cell-edge users are an important issue, and as one means, relay techniques are studied to relay radio transmission between a radio base station apparatus and a mobile terminal apparatus. Using the relay techniques, it is expected to efficiently increase coverage in a place where it is difficult to ensure a wired backhaul link, and the like.
In the relay techniques, there are a layer 1 relay, layer 2 relay and layer 3 relay. The layer 1 relay is a relay technique also called the booster or repeater, and is the AF (Amplifier and Forward) type relay technique for amplifying power of a downlink reception RF signal from a radio base station apparatus to transmit to a mobile terminal apparatus. An uplink reception RF signal from the mobile terminal apparatus also undergoes power amplification similarly and is transmitted to the radio base station apparatus. The layer 2 relay is the DF (Decode and Forward) type relay technique for demodulating and decoding a downlink reception RF signal from a radio base station apparatus, then performing coding and demodulation again, and transmitting the signal to a mobile terminal apparatus. The layer 3 relay is a relay technique for decoding a downlink reception RF signal from a radio base station apparatus, then reproducing user data, in addition to demodulation and decoding processing, performing processing (concealment, user data segmentation and packet concatenation processing, etc.) to perform radio user data transmission again, further performing coding and demodulation, and then, transmitting to a mobile terminal apparatus. Currently, in the 3GPP, standardization of the layer 3 relay technique has proceeded, from the viewpoints of improvements in reception characteristics due to noise cancellation and easiness in standard specification study and implementation.
FIG. 1 is a diagram illustrating the outline of the radio relay technique by the layer 3 relay. A radio relay station apparatus (RN) of the layer 3 relay is characterized by having a specific cell ID (PCI: Physical Cell ID) different from that of a radio base station apparatus (eNB) in addition to performing user data reproduction processing, modulation/demodulation and coding/decoding processing. By this means, a mobile terminal apparatus (UE) identifies a cell B formed by the radio relay station apparatus as a cell different from a cell A formed by the radio base station apparatus. Further, since control signals of physical layers such as a CQI (Channel Quality Indicator) and HARQ (Hybrid Automatic Repeat reQuest) are terminated in the radio relay station apparatus, the mobile terminal apparatus regards the radio relay station apparatus as a radio base station apparatus. Accordingly, mobile terminal apparatuses only having LTE functions are also capable of connecting to the radio relay station apparatus.
Further, it is considered that different frequencies or the same frequency is used to operate the backhaul link (Un) between the radio base station apparatus and the radio relay station apparatus, and access link (Uu) between the radio relay station apparatus and the mobile terminal apparatus, and in the latter case, when the radio relay station apparatus performs transmission and reception processing at the same, unless sufficient isolation is not secured in the transmission and reception circuits, a transmission signal enters a receiver of the radio relay station apparatus and causes interference. Therefore, as shown in FIG. 2, when the same frequency (f1) is used to operate, it is necessary to perform Time Division Multiplexing (TDM) on radio resources of the backhaul link and access link (eNB transmission and relay transmission) to control so that transmission and reception is not performed at the same time in the radio relay station apparatus (Non-patent Document 1). Therefore, for example, in downlink, the radio relay station apparatus is not able to transmit a downlink signal to a mobile terminal apparatus for a period during which a downlink signal is received from the radio base station apparatus.
In addition, in the case that the radio base station apparatus (eNB) transmits a downlink signal to a mobile terminal apparatus (UE) via the radio relay station apparatus (RN), there are an aspect (single hop relay) that the eNB transmits to the UE via a single RN as shown in FIG. 3(A), and another aspect (multi-hop relay) that the eNB transmits to the UE via a plurality (two in FIG. 3B) of RNs as shown in FIG. 3(B).